Expanded grid static mixer

ABSTRACT

A static mixer in the exhaust emission control system of an excess-air-operated combustion engine is formed of an expanded grid with a plurality of openings formed between crossbars. Using an expanded grid achieves both good mixing of the exhaust gas with a reducing agent in a short mixing path and properly aligns the exhaust flow.

This application is a division of application Ser. No. 09/519,384 filedMar. 3, 2000, now U.S. Pat. No. 6,401,449, which is a continuation ofInternational Application PCT/DE98/02780, with an international filingdate of Sep. 18, 1998, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a static mixer for placement in a flowchannel and, more particularly, to an expanded grid static mixer forimproved mixing of exhaust gases and a reducing agent in an exhaustemission control system of an internal combustion engine.

2. Description of Related Art

The use of a regulated or controlled diesel catalytic converter hasproven to be an advantageous technique to reduce pollutants,particularly nitrogen oxides, in the exhaust of an internal combustionengine. This technique is used in particular in internal combustionengines operated with excess air, such as diesel and lean-mix engines.This technique, based essentially on the method of selective catalyticreduction (SCR), brings the nitrogen oxides into contact with a reducingagent on a selective catalyst, which converts them into harmlessnitrogen and water.

An exhaust emission control system operating on this principle isdiscussed in a Siemens Aktiengesellschaft pamphlet entitled “SiNOxNitrogen Oxide Reduction for Stationary Diesel Engines,” Order No.A96001-U91-A232, Siemens AG, Bereich Energieerzeugung (KWU),Freyeslebenstraβe 1, 91058 Erlangen, Germany (1994). Such a system candrastically reduce the nitrogen oxide emissions from a diesel engine. Itmixes the nitrogen oxides in the exhaust gas with a reducing agent,usually ammonia obtained from urea by hydrolysis, in the exhaust pipe,and then introduces the mixture into a reactor space with a catalystdevice for catalytic conversion. A static mixer, such as that shown inDE 41 23 161 (U.S. Pat. No. 5,489,153), is used for mixing the reducingagent with the exhaust gas.

Known static mixers have proven generally satisfactory in homogeneouslymixing reducing agents with the exhaust gas in such exhaust emissioncontrol systems. However, the exhaust pipe configuration in a vehiclemust be adapted to the vehicle's structural design, and in manyapplications the minimum mixing path required by known mixers cannot berealized because of lack of space. In such a case, the flow conditionsinto the catalytic converter may be unfavorable, resulting in an unevenflow profile at the converter inlet.

SUMMARY OF THE INVENTION

It is an object of the present invention to avoid the shortcomings ofprior art mixers in such situations.

The present invention relates to a static mixer for placement in theflow channel of an exhaust emission control system, which seeks toprovide as short a mixing path as possible and align the flow directionof the exhaust gas in the flow channel.

In furtherance of the objects of the present invention, one aspect ofthe invention involves an exhaust system for an internal combustionengine having a catalytic converter for reducing pollutants in theexhaust gases from the engine in the presence of a reducing agent, whichexhaust system comprises an exhaust duct for carrying the exhaust gasesfrom the engine to the catalytic converter, the exhaust duct having aninjection location for the introduction of the reducing agent into theexhaust gases, and a static mixer disposed in the exhaust duct upstreamof the catalytic converter and proximate to the injection location, thestatic mixer comprising an expanded grid in the form of a mesh having aplurality of openings formed between crossbars of said expanded grid.

In accordance with another aspect of the present invention, anexcess-air-operated internal combustion engine comprises a catalyticconverter for reducing pollutants in the exhaust gases from the enginein the presence of a reducing agent, an exhaust duct for carrying theexhaust gases from the engine to the catalytic converter, an injectiondevice in the exhaust duct for introducing the reducing agent into theexhaust gases upstream of the catalytic converter, and a static mixerdisposed in the exhaust duct upstream of the injection device, thestatic mixer comprising an expanded grid in the form of a mesh having aplurality of openings formed between crossbars of the expanded grid.

Another aspect of the present invention is a method of mixing exhaustgases of an internal combustion engine with a reducing agent, the methodcomprising providing a static mixer comprising a conventional expandedgrid in the form of a mesh having a plurality of openings formed betweencrossbars of the expanded grid, and placing the static mixer in anexhaust duct of the engine proximate to an injection location for theintroduction of the reducing agent into the exhaust gases.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are explained in more detail withreference to four figures, in which:

FIG. 1 schematically depicts an exhaust emission control system with SCRcatalytic reduction, having static mixers according to the presentinvention located in an exhaust pipe upstream and downstream of areducing agent injection location.

FIG. 2 is a detailed view of an embodiment of one of the static mixersshown in FIG. 1, with a laminated construction in an inclinedorientation in the exhaust pipe.

FIG. 3 is a perspective view of an embodiment of an expanded grid staticmixer according to the present invention.

FIG. 4 is a section taken along the line IV—IV in FIG. 3.

In the drawings, the same components are given the same referencenumbers or letters in the different figures.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates an exhaust gas A discharged from anexcess-air-operated internal combustion engine (not shown). The enginemay be a diesel engine with an exhaust turbocharger (not shown) thatdirects the exhaust gas A into a flow channel formed by an exhaust pipe2.

The exhaust pipe has therein a first static mixer assembly 18 employingone or more expanded grids 4, discussed in more detail below inconnection with FIG. 3. Following in the direction of flow, an injectiondevice 6 for injecting a reducing agent R into the flow is disposeddownstream of the first static mixer, at a curved section 8 of theexhaust pipe 2. The reducing agent R may be ammonia (NH₃), in particularan aqueous solution of urea. At a transitional point 10, the exhaustpipe 2 widens to form an exhaust-gas catalytic converter compartment 12.A second static mixer assembly 18′ is disposed downstream of theinjection device 6. The second mixer is located in the exhaust pipe 2just upstream of the transitional point 10, and therefore upstream ofthe catalytic converter compartment 12. The second static mixer assembly18′ likewise employs one or more expanded grids 4.

The exhaust-gas catalytic converter compartment 12 is shown as havingthree SCR catalyst modules 14 arranged one behind the other. It servesin a conventional fashion to eliminate nitrogen oxides (NOx) in theexhaust gas A. An example of a suitable catalytic converter is shown inInternational Patent Application WO 97/01387, incorporated herein byreference as if set out in full. The cleaned exhaust gas A′ exits theexhaust pipe 2 via an outlet 16. FIG. 3 is a perspective view of anembodiment of an expanded grid static mixer according to the presentinvention.

FIG. 2 illustrates in more detail the first static mixer assembly 18depicted in FIG. 1. The assembly 18 comprises plural static grids 4, inthis case three grids 4 a, 4 b and 4 c. They are laminated, that is,they comprise a plurality of expanded grids 4 in series one behind theother, fitted into the exhaust pipe 2.

Each static mixer in FIG. 1 may be one expanded grid 4 or an assembly ofplural, laminated expanded grids. In either case, the distance X betweenthe static mixers is preferably less than or equal to five times thediameter D of the exhaust pipe 2.

In comparison with prior art mixers, use of an expanded grid as a staticmixer in an exhaust emission control system of this kind has been foundto be an improvement both in providing more even distribution of theexhaust gas/reducing agent mixture and better alignment of the flowwithin the flow channel or exhaust pipe 2.

For instance, when the SINOx exhaust emission control system is used fora commercial vehicle in which the curved section 8 of the exhaust pipe 2coming from the vehicle engine had to be installed directly ahead of theexhaust-gas catalytic converter compartment 12 on account of theavailable installation space in the vehicle, it was possible to note anappreciable increase in the NOx conversion brought about by theinstallation of the expanded grid 4 as a mixer/flow director upstream ofthe curved section 8. It is believed that this increase in conversion ofNOx in comparison with known mixers can be attributed to the effectexerted by the respective expanded grid 4 on the direction of flowtherethrough. As is known, a pipe bend such as the curved section 8leads to the formation of an asymmetrical flow profile at the injectionpoint 19, determined by the position of the injection device 6, as wellas at the exhaust-gas catalytic converter compartment 12.

As represented in FIG. 2, one or all of the expanded grids making up astatic mixer assembly according to the present invention may be inclinedto the transverse direction 20 of the exhaust pipe 2 at an angle ofinclination α to the longitudinal axis 21 of the pipe. In this case, theangle of inclination α is preferably between 0° and 45°.

An expanded grid used in the present invention may be made of anysuitable material, plastic or metal being typical. A metal grid of thetype suitable for use in the present invention is also sometimesreferred to as an expanded mesh. An expanded grid of this kind isdescribed, for example, in the company brochure “Strecken macht gröβer,Streckgitter” [“Expansion Increases Size, Expansion Grids”] of thecompany Ernst Sorst & Co., Hannover, Germany, and is commerciallyavailable in a variety of configurations. DE 32 08 634 C2 (U.S. Pat. No.4,567,630) discloses a manner of making a suitable expanded mesh sheet,in that instance used as a support for a catalytic substance in avehicle exhaust system. In connection with the present invention, theterms “expanded grid,” “expanded-mesh grid,” and the like refer to amesh formed from a sheet of material by slitting and expanding thesheet, for example, as illustrated in the Ernst Sorst & Co. brochureunder the heading “Streckgitter” or as discussed in U.S. Pat. No.4,567,630 at column 1, lines 18-35, to provide a configuration as shownin FIGS. 3 and 4 hereof. U.S. Pat. No. 4,567,630 and the Ernst Sorst &Co. brochure are incorporated herein by reference as if set out in full.

The individual expanded grids 4 a to 4 c used within a laminatedassembly 18 may also be of different types. For instance, various typesof the expanded grids represented in the company brochure mentionedabove, such as expanded grids with long crossbars and deformed orpleated expanded grids, may be combined within an exhaust emissioncontrol system.

As seen in FIGS. 3 and 4, the expanded-mesh grid 4 comprises a matrix ofopenings 24, which are bounded by crossbars 22 and through which theexhaust gas A flows during operation of the exhaust emission controlsystem. In this case, the effect of the direction of flow is determinedessentially by the crossbar width b. The crossbar thickness d,established by the thickness of the material used to make the expandedmesh, and the length and width of the node points 26, formed by fourmeeting crossbars 22, determine the effect of the mixing. Particularlygood mixing and effective direction of flow are obtained with a ratio ofthe crossbar width b to the crossbar thickness d in the range between0.5 and 5. The free cross-section, that is, the area of all of theopenings 24, should be between 25% and 85% of the total area of theexpanded grid 4 (and thus of the cross-sectional area nD²/4 of theexhaust pipe 2).

In accordance with the present invention, the static mixer alsofunctions as a flow director. During the operation of the exhaust systemand the exhaust emission control system, a reducing agent introducedinto the exhaust emission control system is mixed with the exhaust gason account of the mixing action generated by the expanded grid. At thesame time, the expanded grid aligns the flow at least approximatelyparallel to the longitudinal axis or axis of symmetry of the exhaustpipe 2. Aided by suitable arrangement of the expanded grid, unfavorableflow profiles within the exhaust pipe, induced in particular as a resultof given installation configurations required by vehicle structure, areeliminated, and the flow is made more uniform, all in a particularlysimple and effective way.

As noted above, the expanded grid typically fills the entire crosssection of the exhaust pipe and about 25% to 85% of the expanded grid isopen to the flow. Depending on the application, the expanded grid may bedesigned with a fine or coarse mesh and as an expanded grid with longcrossbars. The expanded grid may be installed normal to the longitudinalaxis of the exhaust pipe, or more preferably, at an angle of inclinationup to 45°.

When the flow passes around the grid nodes or node points of theexpanded grid, vortexes and turbulence are generated to induce mixing.At the same time, the crossbars of the expanded grid align the flow ofthe exhaust gas, or of the exhaust gas/reducing agent mixture, accordingto the angle the crossbars present to the flow. With an increasing ratiobetween the crossbar width and crossbar thickness, an increasing effecton the flow direction is created. With a decreasing ratio, on the otherhand, increased mixing is obtained. As mentioned above, a particularlypreferred range of the ratio between crossbar width and crossbarthickness has been found to be between 0.5 and 5.

At least one of the expansion grids is advantageously arranged upstreamof the reducing agent injection point with respect to the direction offlow of the exhaust gas. This is particularly advantageous if injectiontakes place within a bend of the exhaust pipe, since more exhaust gaswould otherwise occupy the outer region of the exhaust pipe bend than inthe inner region. The resultant unfavorable flow conditions in the pipebend normally lead to the exhaust gas being subjected particularlyunevenly to the reducing agent when the latter is sprayed into theexhaust pipe. That unfavorable flow profile is made more uniform by theupstream expanded grid, so that homogeneous mixing of the exhaust gaswith the reducing agent is provided even when injection takes place ator near a bend in the exhaust pipe. To make the flow profile even moreuniform as the exhaust-gas/reducing-agent mixture enters the reactionspace, a second expanded-grid static mixer is expediently provideddirectly ahead of the catalytic converter.

If a number of expanded grids in a plurality of layers or laminatedassemblies are used, the alignment of the expanded grids in relation toone another is performed initially with a view to particularly goodmixing of the exhaust gas with the reducing agent. In this case, thelast layer downstream also serves for directing the flow according tothe following pipe or channel routing. In the case of a plurality oflayers, different expanded grids with different mesh openings, crossbardimensions and/or node dimensions are preferably used.

Use of an expanded grid as a static mixer in the flow channel of anexhaust emission control system of an excess-air-operated combustionengine provides good mixing of the exhaust gas with a reducing agent,even over a particularly short mixing path. In addition, the flow of theexhaust gas and/or exhaust-gas/reducing-agent mixture is made moreuniform in a particularly simple way because the expanded grid alsoserves as a flow director.

Although preferred embodiments of the invention have been depicted anddescribed, it will be understood that various modifications and changescan be made other than those specifically mentioned above withoutdeparting from the spirit and scope of the invention, which is definedsolely by the claims that follow.

What is claimed is:
 1. An exhaust system for an internal combustionengine having a compartment housing a catalytic converter for reducingpollutants in the exhaust gases from the engine in the presence of areducing agent, the exhaust system comprising: an exhaust duct forcarrying the exhaust gases from the engine to the catalytic convertercompartment, said exhaust duct having an injection location for theintroduction of the reducing agent into the exhaust gases; and a staticmixer disposed in said exhaust duct at a location spaced upstream of thecatalytic converter compartment and proximate to said injectionlocation, said static mixer comprising an expanded grid made of at leastone sheet that has been slit and expanded to provide a mesh having aplurality of openings formed between crossbars of said expanded grid. 2.The exhaust system of claim 1, wherein said static mixer is disposedupstream of said injection location.
 3. The exhaust system of claim 1,wherein said openings comprise between 25% to 85% of a cross-sectionalarea of said exhaust duct.
 4. The exhaust system of claim 1, whereinsaid expanded grid is inclined at an angle a relative to a transversedirection normal to a longitudinal axis of said exhaust duct, and0°≦α≦45°.
 5. The exhaust system of claim 1, wherein said crossbars havea width b and a thickness d, and b/d is between 0.5 and
 5. 6. Theexhaust system of claim 1, wherein said static mixer comprises aplurality of said expanded grids.
 7. The exhaust system of claim 6,wherein said plurality of said expanded grids comprise a laminatedassembly.
 8. An excess-air-operated internal combustion enginecomprising: a catalytic converter for reducing pollutants in the exhaustgases from the engine in the presence of a reducing agent; an exhaustduct for carrying the exhaust gases from the engine to said catalyticconverter, wherein said catalytic converter is housed in a compartmentin said exhaust duct; an injection device in said exhaust duct forintroducing the reducing agent into the exhaust gases upstream of saidcatalytic converter compartment; and a static mixer disposed in saidexhaust duct at a location spaced upstream of said injection device,said static mixer comprising an expanded grid made of at least one sheetthat has been slit and expanded to provide a mesh having a plurality ofopenings formed between crossbars of said expanded grid.
 9. The internalcombustion engine of claim 8, wherein said static mixer comprises alaminated assembly including a plurality of said expanded grids, in eachof which said openings comprise between 25% to 85% of a cross-sectionalarea of said exhaust duct and said crossbars have a width b and athickness d, and b/d is between 0.5 and
 5. 10. The internal combustionengine of claim 9, wherein each said laminated assembly is inclined atan angle a relative to a transverse direction normal to a longitudinalaxis of said exhaust duct, and 0°≦α≦45°.
 11. The internal combustionengine of claim 8 wherein said injection device is disposed in a curvedsection of said exhaust duct and said static mixer is disposed in astraight section of said exhaust duct.